1. Field of the Invention
The present invention relates to an optical pickup device including a plurality of objective lenses having different optical characteristics.
2. Description of the Background Art
Some optical pickup devices include a plurality of kinds of objective lenses having optical characteristics corresponding to different kinds of optical recording medium such as a DVD (Digital Versatile Disc) and a Blu-ray Disc (trademark) in order to reproduce/record information from/to the optical recording medium. In such an optical pickup device, a plurality of kinds of objective lenses are attached to a single lens holder and used while being switched in accordance with the kind of an optical recording medium. Specifically, for example, at the time of reproducing information from a DVD, a laser beam emitted from a light source is condensed by an objective lens adapted to a DVD and the condensed light is emitted to the DVD. At the time of reproducing information from a Blu-ray Disc, a laser beam emitted from a light source is condensed by an objective lens adapted to a Blu-ray Disc and the condensed light is emitted to the Blu-ray Disc.
The working distance between an optical recording medium and an objective lens varies according to the kind of the optical recording medium and the objective lens. For example, the working distance between a Blu-ray Disc having a recording density higher than that of a DVD and an objective lens corresponding to a Blu-ray Disc is much shorter than that between a DVD and a corresponding objective lens because a laser beam having a shorter wavelength is condensed and the condensed laser beam is emitted. If the working distance becomes short to a certain extent, in the case such that focus servo is deviated, an objective lens and an optical recording medium collide with each other and are damaged. Consequently, a buffer member as disclosed in, for example, Japanese Patent Application Laid-Open Nos. 2004-185721 and 2001-67700 is provided near the objective lens having the shorter working distance, thereby preventing collision between the objective lens and an optical recording medium.
In a light spot formed by condensing a laser beam by an objective lens onto an optical recording medium, coma aberration occurs due to a manufacture error in the objective lens or an error of attachment to a lens holder. When coma aberration becomes large to a certain degree, the reproduction performance and the recording performance deteriorate. Consequently, as disclosed in the paragraph [0002] in Japanese Patent Application Laid-Open No. H10-154342 and the paragraph [0003] in Japanese Patent Application Laid-Open No. H11-23960, by adjusting the angle of attaching the lens holder to a fixed member, the tilt of an objective lens fixed to the lens holder is changed to correct coma aberration.
However, in the case of fixing a plurality of kinds of objective lenses in a single lens holder, the objective lenses are not always fixed by the lens holder so that the tilting directions of the objective lenses capable of eliminating coma aberration which occurs in the objective lenses become the same. Consequently, even when the objective lenses in the lens holder are tilted in the same direction to correct coma aberration which occurs in one of the objective lenses, there is the possibility that coma aberration which occurs in the other objective lens becomes very large. In Japanese Patent Application Laid-Open No. H10-154342, the first objective lens is mounted and fixed on a first bearing surface so that the direction of a tilt of the first bearing surface with respect to a second bearing surface provided for the lens holder and the direction of coma aberration in the first objective lens are opposite to each other, thereby suppressing worsening of the coma aberration occurring in the first objective lens due to elimination of the coma aberration in the second objective lens simply fixed to the second bearing surface (without considering the direction of coma aberration). Japanese Patent Application Laid-Open No. H11-23960 discloses design specifications and parameters of the objective lenses for suppressing worsening of coma aberration occurring in one of the objective lenses caused by elimination of the coma aberration in the other objective lens.
However, Japanese Patent Application Laid-Open Nos. H10-154342 and H11-23960 disclose only the conditions of attaching the objective lenses to the lens holder and the design specifications and parameters of the objective lenses. Therefore, the techniques cannot properly correct coma aberration caused by a relative tilt error between the objective lenses when components of the optical pickup device are actually combined.
The present invention has been achieved to solve the problem and an object of the invention is to provide an optical pickup device capable of properly suppressing coma aberration caused by a relative tilt error between objective lenses.